This invention relates to the removal of molybdenum from uranium-bearing solutions. Typically these solutions contain high concentrations of uranium and low concentrations of molybdenum.
Extraction of uranium from its ores is commonly carried out by processes which include leaching the ore or a concentrate thereof. Many uranium ores also contain molybdenum and, in such cases, leaching yields a solution which contains both uranium and molybdenum along with other impurities such as iron, aluminum, calcium, colloidal silica, etc. The dissolved uranium is usually separated from the leach solutions by an ion exchange or solvent extraction process. This produces a purified uranium-containing solution from which uranium is recovered by precipitation.
It is often difficult, however, to obtain an uncontaminated uranium product when the leach solution also contains dissolved molybdenum. The molybdenum is coextracted with the uranium in both the ion exchange and solvent extraction processes and is subsequently coprecipitated with the uranium.
Various proposals have been made and employed by the industry to deal with the problem of molybdenum contamination, but such proposals have various disadvantages. For example, both uranium and molybdenum may be stripped from the loaded resin or extractant by sodium carbonate solution and the uranium may be selectively precipitated by sodium hydroxide. Although the precipitated uranium product obtained is relatively free of molybdenum, the sodium content in the product can render it undesirable. Furthermore, the presence of sodium in the resulting effluent can present an environmental problem.
Another proposal is to selectively strip uranium from a loaded resin or extractant by an acidified sodium or potassium chloride solution. Although adequate separation of uranium and molybdenum can be achieved in this way, the resulting effluent of this process contains an undesirable amount of sodium chloride.
U.S. Pat. No. 4,405,566 (Weir et al.) discloses a process for recovering uranium values from a sulphate solution containing dissolved uranium and molybdenum and with a pH not exceeding about 5.5. It includes reacting the solution with ammonia at a pH in the range of from 8 to about 10, with resultant precipitation of uranium values relatively uncontaminated by molybdenum. This process, however, requires large amounts of ammonia and elaborate ventilation facilities.
The most commonly used method of extracting uranium from an ore is a sulphuric acid and oxidant leach process followed by either filtration or counter-current decantation washing of the leach residues. Ion exchange or solvent extraction is used to selectively recover uranium from the leach solution. Subsequently, the uranium is stripped from the loaded resin or extractant and precipitated and separated from the strip solution as a solid uranium compound. If the ore and subsequent leach solution contain molybdenum, it is extracted, stripped and precipitated together with the uranium, thus contaminating the uranium product.
A typical extraction process, involving solvent extraction, is carried out as follows. The uranium-containing ore is leached with sulphuric acid to produce an acidic solution containing dissolved uranium and impurities. The acidic solution, which is an aqueous phase, is then mixed with an immiscible amine phase. The amine phase is comprised of a trialkylamine, for example Alamine 336 (trade-mark), dissolved in kerosene and a small amount of isodecanol. Mixing of the aqueous and amine phases exposes the amine phase to most of the uranium dissolved in the aqueous phase. Since the uranium has a greater affinity for the amine phase than the aqueous phase, the uranium, but not most of the impurities, is extracted from the aqueous phase into the amine phase. After mixing, the mixture is allowed to settle whereby the amine phase containing dissolved (extracted) uranium separates from the aqueous phase by rising to the top of the aqueous phase. The amine phase is then removed, leaving behind the aqueous phase containing impurities which were not extracted by the amine phase. Since the extraction by the amine phase does not completely remove all of the uranium from the aqueous phase, the aqueous phase remaining after extraction is subjected to further similar extraction steps, each of which removes more uranium from the aqueous phase. After four such extraction steps, over 99% of the uranium has been extracted from the aqueous phase.
The amine phase containing dissolved uranium and now fewer impurities is then subjected to an extraction step with a weak acid solution in order to remove arsenic impurities. After the amine phase has settled away from the acid solution containing extracted arsenic, the amine phase is subjected to a further extraction step wherein a slightly acid solution of ammonium sulphate is mixed with the amine phase. This extracts the uranium. After mixing, the acid solution phase separates from the amine phase. The acid solution containing the extracted uranium is referred to as the "loaded strip solution". This is treated with ammonia to precipitate uranium yellowcake product. Such product is, however, contaminated with molybdenum if the ore contained molybdenum.